1. Field of the Invention
The present invention relates to an apparatus and method for air cooling printed circuit board modules which minimizes the spacing of the boards and minimizes the associated sound.
2. Description of the Prior Art
Devices for cooling printed circuit boards are well known. Conventional methods utilize either air cooling across the faces of the boards or liquid cooling circulating liquid within a plate attached to the board when configured as a board module. Liquid cooling has been used for most, but not all, super computer applications as previous air cooling techniques could not accommodate three-dimensional packing density with short signal path lengths required by the high frequency and high integrated circuit count super computers. However, liquid cooling has several drawbacks as there are higher facilities cost for heat exchange units and refrigerant condensing units which are required. In addition, liquid cooling also has a higher operating cost due to higher electrical power needs and substantially more mechanical parts. Liquid cooling requires more floor space and higher initial costs due to increased associated mechanical machinery. Liquid cooling also uses expensive and highly evaporative dielectric coolants which pose safety hazards, thus requiring additional safeguards for handling the coolant liquid.
Although air cooling may be preferred for the reasons stated above, there are several drawbacks that must be overcome to provide adequate air cooling. Prior systems have used a centrifugal blower, but the centrifugal blowers cannot provide the pressure required using miniature heat sinks and also have a very high temperature rise as the air passes through the blower. Use of rotary positive displacement blowers provides sufficient pressure, flow rate, and reduced temperature rise, but creates different problems regarding noise control. Positive displacement blowers have noise in lower frequencies, in the range of 125-250 hertz, which must be silenced with a combination of reactive and dissipative silencers. Desired computer room noise levels in the range of 75 decibels could not be obtained using standard commercial silencers even when used in a tandem arrangement. To achieve improved noise levels, a larger volume silencer was required and/or large pressure drops would be experienced. In computer room environments, space is very valued and additional space required for the silencing equipment can often not be spared.
Due to the high costs involved in operating super computers and other high density integrated circuits, there must be provided oversizing and redundancies to ensure that system downtime is reduced to a minimum.
In addition to providing space for external cooling machinery, the boards or modules must be sufficiently spaced to allow enough air flow across the boards to carry the excess heat away. There are applications wherein high density is important. Board density with air cooling may be lower than that required to obtain comparable cooling capacity from a liquid cooled system. This increases the volume of the stack of boards or board modules due to the increased spacing between the boards.
Air cooling also has problems associated with uneven flow and pressures. Entrance ends of the cooling area are likely to have less flow than an opposite end of the chamber and the mid portions of the chamber. Attempts to equalize the flows and pressures across an area typically require increasing the size of manifolds or employing complicated duct systems. Such uneven cooling leads to oversizing of the cooling system to accommodate localized hot spots in the boards which may occur due to uneven flow and eddies which may form in the air currents
The problem of providing adequate even cooling without excessive size and control is increased when control valving is required Flow may need to be varied due to increased heat generation or changes in the surrounding environment.
It can be seen then that an improved apparatus and method for cooling stacks of circuit board modules are required which provide adequate cooling and minimize the associated noise and space required for ventilation. It can also be seen that it would be advantageous to utilize air for cooling rather than liquids if comparable board densities and comparable cooling are obtained.